1. Field of the Invention
The present invention relates generally to shields for protecting antennas from undesirable radio frequency interference. More particularly, the present invention relates to a shield for protecting a satellite earth station antenna from interference in dense radio frequency environments.
2. Description of the Related Art
Generally, in any communication link between a receiver and a transmitter, proper shielding is required to prevent undesirable external signals from degrading the communication signal. In the case of satellite communication between an earth station antenna and a satellite transponder, it is important that the earth station be adequately protected from undesirable radio frequency signals surrounding its particular location on the earth.
In order to minimize the effect of harmful interference, it is sometimes desirable to situate an earth station at an appreciable distance from the urban point or points it is to service so that sufficient isolation exists between potential interfering communication systems which share the same frequency spectrum. This is particularly true in regions where there are few hills or valleys to provide natural site shielding for the earth station. If it is impractical or otherwise undesirable to locate an earth station in a remote area, artificial site shielding must be used to provide insulation between potentially interfering stations.
If an earth station is to be placed in an urban environment, it must not only be protected from radio frequency interference, but it must be designed so that it does not transmit appreciable interference into terrestrial stations of other carriers (i.e., competitors) in the vicinity. The other carriers in the area may effectively veto an FCC license for an earth station designed with inadequate shielding.
These competitors also reserve the right to perform interference measurements after the earth station facility has been built. If measurements show that interference is being transmitted to surrounding terrestrial stations, the owner of the new facility is required to eliminate that interference. Thus, a very high degree of risk is involved in building an earth station that may not meet the necessary interference standards. Fixing or eliminating such interference, once a structure is built, can be extremely expensive, if not impossible.
One manner of shielding an earth station antenna in a dense radio frequency environment has been to try to take advantage of the urban "landscape" by locating the antenna between tall buildings in such a manner so as to utilize the natural shielding provided by the buildings. However, such a location is not always available or desirable or positioned in the optimum fashion. Additionally, the typical building surfaces of such structures can tend to reflect towards the antenna undesirable interference signals that manage to enter the zone of the "natural" shield.
Metal fences have also typically been used to shield earth station antennas from radio frequency interference. These highly reflective fences are usually constructed of a metal screening having a mesh size which is a significantly small fraction of the wavelength of interest to limit energy transmission through the fence to an acceptable level. While such fences are an inexpensive solution to eliminating radio frequency interference, they are susceptible to adverse weather conditions and may require periodic maintenance. Moreover, while metal fences may perform quite well in less dense radio frequency environments, they do not always provide adequate protection in more demanding situations. For example, in a shield constructed of a metal fence that surrounds the antenna, energy diffracting over the top of the fence is projected against the opposing fence inner surface and is reflected toward the earth station antenna. Conversely, stray signals transmitted by the earth station may cause interference to other carriers through the opposite propogation path.
Another method widely used to protect earth station antennas has been to place the antenna in a deep ground depression or pit. An article by Edward F. Lucia, Jr. entitled "Artificial Site Shielding For Communications Satellite Earth Stations," IEEE Transactions on Aerospace and Electronic Systems, Volume AES-6, Number 5, September 1970, discloses the use of pit shielding for microwave earth station antennas. While such pits provide adequate shielding, even in dense radio frequency environments, they must be quite deep if they are to provide an adequate shield for the antenna. Such deep pits require a substantial amount of excavation and hence may be prohibitively expensive in many instances. Moreover, since such pits extend a substantial distance into the earth, they may be accompanied by severe water problems, such as accumulation of rainwater in the pit or underground water seepage into the pit.
Another common means for reducing radio frequency interference is to build an earthen berm around an earth station. While the use of a berm is effective in reducing radio frequency interference, such a berm must be built quite high to provide adequate shielding. The construction of a berm of adequate height requires a great deal of earth fill, thereby substantially escalating the cost of the earth station facility.
Moreover, since the slope of either a deep pit or a tall earthen berm is limited due to structural retaining considerations, both alternatives can require a large tract of land, which is frequently unavailable or too expensive at the desired site.
The combination of a metal fence positioned on top of an earthen beam is disclosed as prior art in U.S. Pat. No. 3,495,265 to Phillip H. Smith, entitled "Dielectric Clutter Fence". However, as discussed in the initial portion of the Smith patent, such a shield requires a high metal fence which is expensive to construct and is not completely satisfactory in reducing undesired electromagnetic energy from reaching the antenna. Alternatively, Smith proposes a dielectric fence which does not prevent the interfering signals from impinging on the antenna, but relies instead on being able to intecept and reverse the phase by 180.degree. of 50 percent of the interfering energy, thereby attempting to cancel interference at the antenna. While this approach may be practical for a single frequency with a fixed arrival angle, it does not lend itself to multiple frequency ranges or to interfering signals with varying angles of arrival, as is typically the case in an earth station environment.